docs(volo): add keywords and description

[GuangmingLuo]

What type of PR is this?

Check the PR title.

• [x] This PR title match the format: \<type>(optional scope): \<description>
• [x] The description of this PR title is user-oriented and clear enough for others to understand.

(Optional) Translate the PR title into Chinese.

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en: add keywords and description zh(optional): 给 volo 的文档完善 keywords and description

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